Conceptual Approach to Renewable Barrier Film Design Based on

ARTICLE pubs.acs.org/Biomac

Conceptual Approach to Renewable Barrier Film Design Based on Wood Hydrolysate Y. Z. Zhu Ryberg, U. Edlund, and A.-C. Albertsson* Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ABSTRACT: Biomass is converted to oxygen barriers through a conceptually unconventional approach involving the preservation of the biomass native interactions and macromolecular components and enhancing the effect by created interactions with a co-component. A combined calculation/ assessment model is elaborated to understand, quantify, and predict which compositions that provide an intermolecular affinity high enough to mediate the molecular packing needed to create a functioning barrier. The biomass used is a wood hydrolysate, a polysaccharide-rich but not highly refined mixture where a fair amount of the native intermolecular and intramolecular hemicelluloses-lignin interactions are purposely preserved, resulting in barriers with very low oxygen permeabilities (OP) both at 50 and 80% relative humidity and considerably lower OPs than coatings based on the corresponding highly purified spruce hemicellulose, O-acetyl galactoglucomannan (AcGGM). The component interactions and mutual affinities effectively mediate an immobilization of the chain segments in a dense disordered structure, modeled through the Hansen’s solubility parameter concept and quantified on the nanolength scale by positron annihilation lifetime spectrum (PALS).

’ INTRODUCTION It would be useful to adopt a different way of thinking in macromolecular design to realize the future demands from an increasingly environmentally conscious society on renewable materials with customized performance in demanding applications like packaging. Understanding, creating or maintaining, and manipulating native interactions in biomacromolecular matrices offer a potent approach to the design of functional renewable materials that match and, at least to some extent, may replace the oil-based bulk plastics in a variety of high-volume applications. Key aspects include a material composition inspired by the natural matrices, produced under benign conditions from economically viable resources. Among available renewable macromolecular resources, nonedible biomass, such as that derived from the forestry industry, stands out as abundant, inexpensive and ethically noncontroversial. From these perspectives, hemicelluloses, a family of heterogeneous polysaccharides present in all higher plants, represents a promising resource and have accordingly been gaining increasing attention regarding their recovery and upgrading into highly purified fractions, as well as their utilization as a macromolecular component in the formulation of bulk and higher value applications, including films,1-7 coatings,5 hydrogels,8,9 and hybrid thermoplastics.10 Hemicelluloses, like most polysaccharides, have been known for a long time to present good barrier properties toward oxygen,11 rendering them interesting for renewable packaging applications given that oxygen and water vapor barrier properties are essential protection parameters in food packaging.12 However, their performance is severely limited by their hydrophilicity, deteriorating the barriers at higher relative humidity. A key question for realizing their future as r 2011 American Chemical Society

packaging materials is how to engineer such an inherently hydrophilic resource to function in and sufficiently tolerate environments of higher relative humidity with preserved good barrier properties. A number of attempts, including chemical modifications,2,13-15 have been reported. We propose an innovative and quite different approach to achieve an oxygen barrier, considering the function of native hemicelluloses in the wood fiber cell, which is generally regarded as composed of macromolecules with reciprocal limitations in intermolecular interactions. Cellulose and lignin are incompatible while hemicellulose chains lend themselves to a compatibilizer role with adequate compatibility of the sugar backbones toward cellulose and the selective compatibility of branched segments, like acetylated groups, toward lignin allowing for strong interactions between the hemicelluloses and lignin. A ternary composite with impressive strength, yet flexibility, and hydrophobicity, yet able of confined water transport, is created from the dissimilar constituents. It has even been proposed that native lignin and hemicellulose interactions are of a covalent nature in the wood structure, so-called lignin-carbohydrate complexes (LCC).16,17 The core idea of this work is to preserve and utilize a fair amount of these interactions by not using one or a mixture of refined carbohydrates from wood as the functional component of the barrier material product. Instead, we use an only partly upgraded mixture,18,19 a wood hydrolysate isolated from the process or wastewater of a wood refining process (e.g., the Received: January 27, 2011 Revised: February 9, 2011 Published: March 02, 2011 1355

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Biomacromolecules pulping processes). The wood hydrolysate, containing hemicelluloses as a major component, is separated from the cellulose component but still purposely contains a fair amount of the lignin-carbohydrate interactions, hypothesizing that they contribute to hindering diffusion of a gas like oxygen through the matrix. To enhance the mechanical properties and obtain the proper balance of the network of interactions between constituent molecules, a co-component is added. The nature of such a cocomponent must be carefully chosen so that it is renewable and bears functional groups such that primary and secondary interactions between the wood hydrolysate and the co-component can be formed. The number and nature of hydrolysate-cocomponent interactions are hereby actively engineered to get the sought-after barrier property. A combined theoretical and experimental approach is presented to shed light and provide a molecular length scale fundamental understanding on how the interactions between wood hydrolysate components, with a co-component that strengthens the intermolecular interactions, should be manipulated to provide a molecular packing suitable for creating an adequate oxygen barrier material. The immobilization of the chain segments in a dense disordered structure arises from strong component mutual affinities, hence, a calculation of group contribution based on the Hansen’s Solubility Parameter (HSP) theory20 is adopted to reveal the interaction driven compatibility between hydrolysate-based barrier matrix components. For a multicomponent system, polymer segments of high mutual affinities locate near each other in the resulting matrix, immobilizing the chains and leading to a lower free volume defined as voids or small holes in the matrix.12 Obviously, lower free volume is a key to a decreased mobility of small permeants like oxygen through the matrix. Positron annihilation lifetime spectrum (PALS) uses positronium in its ortho-state (spin 1), oPs, as a probe to provide direct insight into the size and distribution of free volume of amorphous systems like the matrices herein proposed and is the most prominent method to estimate size distribution of subnanometer size local free volumes.21-24 Our aim is to design a class of wood hydrolysate formulations, inspired by the native molecular interactions, which work as renewable oxygen barrier materials with functional properties arising from their inherent and purposely created intermolecular interactions and improved molecular packing structure.

’ EXPERIMENTAL SECTION Materials. A wood hydrolysate was produced from spruce chips, kindly supplied by S€odra Cell AB, according to a recently invented upgrading procedure involving the recovery of wastewater generated in the pulping process.18,19 Highly purified acetylated galactoglucomannan (AcGGM) from spruce was recovered from thermomechanical pulping process water via fractionation by ultrafiltration. The average molecular weight (Mw) was about 7500 g/mol, the PDI was ∼1.3, and the degree of acetylation (DSAc) was 0.30, as determined according to a published protocol.25,26 Carboxymethyl cellulose sodium salt (CMC), having a degree of substitution of 0.6-0.9 and a medium viscosity of 400-1000 mPa 3 s in a 2% H2O solution at 25
C, was purchased from SigmaAldrich. Chitosan from crab shells was used as received from Fluka with a molecular weight of 150000 g/mol and degree of deacetylation of 80%. Acetic acid was purchased from BioUltra >99.5% (GC/T) and used as received. Commercially used polyethylene terephthalate (PET) films were kindly provided by Tetra Pak Packaging Solutions AB.

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Film Preparation. Films were prepared from wood hydrolysate or highly purified AcGGM mixed with a polysaccharide co-component: either CMC or chitosan. Solutions of each component in varying concentrations were first prepared separately on a shaking board. Wood hydrolysate, AcGGM, and CMC were dissolved in deionized H2O, while chitosan was dissolved in 1% (v/v) acetic acid. Then, the wood hydrolysate or AcGGM solution was mixed with either a CMC or a chitosan solution in varying ratios on a shaking board. The mixtures containing chitosan were additionally homogenized ultrasonically. The final mixtures, having a volume of 28 mL, were casted on Petri dishes (diameter 8.7 cm) and dried for 3 days in a controlled humidity room (55% relative humidity (RH) at 23
C). Coating Preparation. Mixtures of wood hydrolysate or AcGGM with co-components were prepared in exact same way as the mixtures for film preparation. A brush was used to paint the mixtures onto PET films. The coatings were dried in a condition room (50% RH, 23
C) for 48 h. Characterization. Size exclusion chromatography (SEC) was performed on a system consisting of a Waters 515 HPLC pump (Milford, MA), a Rheodyne 7725i (Rohnert Park, CA) manual injector, three TSK-gel columns (Tosoh Bioscience, Tokyo, Japan) coupled in a series, G3000PW (7.5
300 mm, 10 μm particle size), G4000PW (7.5
300 mm, 17 μm particle size), and G3000PW, a Waters 410 Refractive Index (RI) detector, and a Waters 2487 dual wavelength absorbance detector (Milford, MA). The mobile phase was 10 mM NaOH. Polyethylene glycol (PEG) and polyethylene oxide (PEO) standards that have specific molecular weights ranging from 1500 to 400000 g/mol were used for calibrating the columns. After injection of sample solution, the UV absorbances at 280 and 200 nm were recorded together with the RI. The absorption peaks at these wavelengths were analyzed by software Millenium 2. Fourier transform infrared spectrometry (FTIR) was performed on a Perkin-Elmer Spectrum 2000 FTIR with an Attenuated Total Reflectance (ATR) crystal accessory (Golden Gate). All spectra are calculated averages of 16 individual scans at 2 cm-1 resolution in the 4000600 cm-1 interval with corrections for atmospheric water and carbon dioxide. Oxygen permeability measurements of coatings were carried out by Mocon Oxtran 2/20 (Modern Controls Inc., Minneapolis, MN) equipped with a coulometric sensor following ASTM standard D3985-8.27 Samples were cut into suitable size and sealed in aluminum foil with a round open area of 5 cm2 at atmospheric pressure (760 mmHg). The measurements were conducted at two different RH, 50 and 80%, respectively. Sample thicknesses were determined by a Mitutoyo micrometer by taking the average of five discontinuous spots. Each sample was conditioned at 50% RH for at least a week before testing. Thermal gravimetric analyses (TGA) were conducted by using a Mettler Toledo TGA/SDTA 851e. Samples were carefully cut into small pieces of around 5 mg each and then loaded in ceramic cups. The samples were heated from 30 to 600
C at rate of 10
C/min with 50 mL/min N2 flow (AGA Gas AB Sweden, purity > 99.5%). The data was collected by Mettler-STARe Evaluation software and analyzed by Origin 7.0. Positron annihilation lifetime spectrum (PALS) measurements were conducted at room temperature in air using a fast-fast coincidence system21 with a time resolution of 298 ps (FWHM, 22Na) and an analyzer channel width of 25.7 ps. The positron emitter was 1.2 MBq 22 Na2CO3 of high specific activity deposited between two 2 mg/cm2 thin aluminum foils. This source was placed between two identical sample sheets consisting of 8
8 cm film squares stacked into a thickness of 1.1-1.2 mm sufficiently thick to stop all positrons. The apparatus was equilibrated 1 h before the counting was started. Three repeating experiments were conducted for each sample. For each spectrum, 5.7-5.9
106 counts were collected. To correct for source 1356

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contribution, a defect-free p-silicon reference sample (τSi = 219 ps) was measured which resulted in lifetimes of 0.160 ns (Al-foil) and 0.385 ns (22Na2CO3) with a relative sum intensity of 11.25%. This information was used for calibration.28 The mean atomic number of each sample was calculated from the sum of atomic numbers and the number of atoms in the repeating unit for the correction to a lower back-reflection of positrons toward the source. Calculations are based on a modern routine LifeTime, version 9.029 (a calculation routine for fitting the data) and the following equations to estimate the positron lifetime spectrum s(t): Z ¥ Ii Ri ðλÞλ sðtÞ ¼ I1 λ1 expð - λ1 tÞ þ i ¼ 2, 3 0

∑

∑

expð - λtÞdλ with and

i ¼ 1, 2, 3

Ii ¼ 1

"

Ri ðλÞλdλ ¼

1 σi ð2πÞ1=2

# ðln λ=λi0 Þ2 exp dλ 2σi 2

ð1Þ

ð2Þ

where Ri(λ) is the probability density function (pdf) of the annihilation rate of the decay channel i, τi is the characteristic lifetime, and Ii denotes the relative intensity. A nonlinear least-squares fit of the s(t) function, convoluted with the resolution function, to the spectra provides the following annihilation parameters: τ1 = 1/λ1, the position of the maximum λi0 and the standard deviation σi* = σi(λ) of the distributions Ri(λ) (i = 2, 3), the relative intensities Ii, the time zero t0, and the background B of the spectrum. The background B can be estimated from the counting rate per channel at high delay times t = 30-35 ns. The o-Ps, which first is trapped in the free volume (hole) and by collision with the wall of holes after injection into the material, eventually annihilates with a molecule in the hole wall (pick-off annihilation). Due to the collisions, the lifetime of o-Ps will decrease to an extent that correlates to the size and size distribution of subnanometer-size holes in the material. Thus, size of free volume, the hole radius rn in the sample films can be related to the lifetime of o-Ps pick-off annihilation τ3 by the quantum-mechanical (Tao-Eldrup) standard model,30,31 assuming the hole is spherical in shape: "  # rn 1 2πrn -1 sin ð3Þ 1þ 1=τ3 ¼ 1=τpo ¼ λpo ¼ 2ns rn δr 2π rn þ δr where τpo and λpo are the lifetime and rate of o-Ps pick off annihilation, δr, which equals 1.66 Å, is an empirical parameter which describes the wave function of Ps penetrating the hole walls.31,32 Using n(rn) = R3(λ)dλ/drnπ, the distribution of hole radius, n(rn) can be calculated as follows: "  # 2δr 2πrn R3 ðλÞ 1 cos ð4Þ nðrn Þ ¼ rn þ δr ðrn þ δr Þ2 where R3(λ) stands for the o-Ps annihilation rate distribution, determined by eq 2. According to eq 4, hole volume distribution, g(vn) can be obtained by gðvn Þ ¼ nðrn Þ=4πrn2

ð5aÞ

The above distribution is volume-weighted, which presents the hole volume according to the volume between vn and vn þ dvn.21,33 A number-weighted distribution, gn(vn) can also be calculated by gn ðvn Þ ¼ gðvn Þ=vn

ð5bÞ

Hansen’s Solubility Parameter Calculation. The Hansen's solubility parameter (HSP)34 concept regards the total cohesion energy for a molecule as a sum of three individual intermolecular interactions; the

nonpolar forces (D), the dipole-dipole interactions (P), and the hydrogen bonding (H) so that the solubility parameter, δ, may be expressed in accordance with eq 6: δ2 ¼ δD 2 þ δP 2 þ δH 2

ð6Þ

δ is visualized as a point in a three-dimensional space with the Hansen parameters as the x-, y-, and z-axis, respectively. A solute or blend component with high affinity for this particular molecule should have cohesion energy parameters in the vicinity of that of the molecule, typically visualized as a “tolerance sphere” in the three-dimensional space with a radius, R0, which defines the boundaries of deviation allowed. The difference, Ra, in solubility parameters between the two molecules to be mixed, denoted P and S respectively, is quantified by eq 7 and in the ideal case thus is within R0 to represent high mutual affinity. R0 will be larger if swelling, and not only true miscibility on the molecular level, is allowed. ðRa Þ2 ¼ 4ðδDp - δDs Þ2 þ ðδPp - δPs Þ2 þ ðδHp - δHs Þ2

ð7Þ

In this work, the Hansen’s solubility parameters, δD, δP, δH, and δ, were calculated for the repeating unit of each principal component in the system: AcGGM, lignin, CMC, and chitosan using the group contribution method. For wood hydrolysate, the volume fraction of AcGGM and lignin, 9.1:0.9, is taken into account. The AcGGM chains in the wood hydrolysate, recovered as described,19 have an average degree of acetyl substitution of 0.6. For purified AcGGM, the degree of acetyl substitution is 0.3. In the case of CMC, an average carboxymethyl degree from 0.65 to 0.9 is used for the calculation. For the chitosan component, its degree of deacetylation, 0.8, was taken into account. Equation 7 was used to calculate the correlation between different components.

’ RESULTS AND DISCUSSION Presented here is an innovative approach to the design of renewable barrier films based on noncellulosic forestry biomass. Unlike the conventional route using one or a mixture of refined carbohydrates from wood as the functional component of the barrier material product, we seek to preserve a significant amount of the native intermolecular and intramolecular hemicelluloseslignin linkages by using a mixture that is only partly purified, a wood hydrolysate isolated from the process or wastewater of a wood refining process (e.g., the pulping process), separated from cellulose yet polysaccharide rich, containing lignin and purposely still containing a fair amount of the native lignin-carbohydrate complexes. The hypothesis is that these bonds, and the molecular packing they give rise to, contribute to hindering diffusion of a gas like oxygen through the matrix. The hydrolysate was mixed with a co-component, typically a polysaccharide, selected according to its ability to add adequate mechanical performance and forming strong secondary bonds to the oligo- and polysaccharides in the hydrolysate. Again, the hypothesis is that these bonds, and the molecular packing they give rise to, will further enhance the density of the matrix and, thus, give rise to better oxygen barrier performance, even at higher relative humidity, than can be expected for such a hydrophilic matrix material. The hydrolysate used herein is derived from spruce wood and the major hemicellulose component is accordingly O-acetyl galactoglucomannan (AcGGM). The composition (by weight) of the wood hydrolysate is 88.8% oligo- and polysaccharides, 9.1% lignin, 2.1% monosaccharides and 0.1% ash. The molecular weight (Mw) is 3100 g/mol.18 This hydrolysate is utilized to develop oxygen barrier films and coatings for potential package applications. To allow for comparison and shed light on the function of the hydrolysate composition vis-a-vis a refined 1357

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Table 1. Oxygen Permeability (OP) and Oxygen Transmission Rates (OTR) for Coatings Based on Wood Hydrolysate or AcGGM and Co-Components at 50% and 80% Relative Humidity and Reference Values for Polysaccharide Films Reported from Literature coating composi-

average thickness

50% RH OTR

50% RH OP

80% RH OTR

80% RH OP

tiona on PET film

(μm)

(cm3 m-2 day-1)

(cm3 μm m-2 day-1 kPa-1)

(cm3 m-2 day-1)

(cm3 μm m-2 day-1 kPa-1)

wood hydrolysate þ CMC

38. 0 ( 0.4

38.9 ( 5.0

14.6

41.8 ( 0.2

15.7

48.1 ( 8.5

3.4 ( 1.0

1.6

24.3 ( 2.3

11.5

wood hydrolysate þ chitosan

43.5 ( 1.3

7.7 ( 1.9

3.3

24.1 ( 2.9

10.3

AcGGM þ CMC AcGGM þ chitosan

42.0 ( 0.68 43.6 ( 2.0

21.1 ( 1.4 21.8 ( 2.5

8.7 9.4

33.5 ( 1.0 32.1 ( 1.8

13.9 13.8

a

All formulations are based on a co-component amount of 50% (w/w) and 0.4 g/14 mL concentrations, except that the wood hydrolysate/chitosan sample contained a co-component amount of 40% (w/w) due to inhomogeneity when using higher co-component amounts.

system, corresponding formulations are also prepared from the highly purified hemicelluloses AcGGM. Oxygen permeability is a key consideration in the packaging industry to keep food fresh for a longer period of time. An oxygen permeability below 38.9 cm3 μm m-2 day-1 kPa-1 is generally considered as a good barrier for food packaging films.35 Packaging based on coating PET films with the wood hydrolysate described herein exhibit lower oxygen permeabilities than this threshold value and, importantly, perform considerably better than the corresponding formulations based on a highly purified hemicellulose from spruce, as shown in Table 1. Wood hydrolysate as well as AcGGM based coatings significantly decrease the oxygen permeability compared to the PET reference films, both at moderate and high relative humidity (RH). It is noteworthy that wood hydrolysate based coatings show a more dramatic decrease of oxygen transfer rate (OTR) and oxygen permeability (OP) compared to AcGGM at both 50% and 80% RH. Wood hydrolysate coatings based on CMC as a co-component have a lower OTR than chitosan containing coatings, while the co-components give rise to similar OTR values in coatings based on AcGGM. The wood hydrolysate/ CMC coatings show the lowest OP among the developed formulations: 1.6 cm3 μm m-2 day-1 kPa-1 at 50% RH and 11.5 cm3 μm m-2 day-1 kPa-1 at 80% RH, while the corresponding AcGGM/CMC coating shows a much higher value, 8.7 cm3 μm m-2 day-1 kPa-1 at 50% RH, and a slightly higher OP, 13.9 cm3 μm m-2 day-1 kPa-1, at 80% RH. An established threshold value for OP in the food packaging field is an OP of 38.9 cm3 μm m-2 day-1 kPa-1, below which a material is considered as a good barrier.35,36 All the coatings show higher permeability at 80% than at 50% RH, which is as expected considering the hydrophilic nature of the films. It is noteworthy that, even at 80% RH, the wood hydrolysate based coatings still improve the OP of noncoated PET films (15.7 cm3 μm m-2 day-1 kPa-1) to about 10.3 cm3 μm m-2 day-1 kPa. AcGGM based coatings reduce the value only to about 13.8 cm3 μm m-2 day-1 kPa-1. As free-standing films, wood hydrolysate based formulations show the same potential with OP values in the order of 0.3 cm3 μm m-2 day-1 kPa-1 for films with CMC as the co-component and 10 cm3 μm m-2 day-1 kPa-1 for chitosan co-component films.18 The OP value of CMC co-component wood hydrolysate based films is good compared with films from pure hemicellulose based films. A film3 composed of pure AcGGM and 35% CMC shows an OP of 1.28 cm3 μm m-2 day-1 kPa-1 and films made from arabinoxylan with 40% sorbitol gives an OP of 4.7 cm3 μm m-2 day-1 kPa-1.6

Scheme 1. Representative Structures of (a) the Repeating Segment of AcGGM and (b) Mono Lignols

Clearly, the wood hydrolysate, compared to related polysaccharides of comparable hydrophilicity, shows an unconventionally strong ability to effectively contribute to oxygen barrier properties even under high humidity conditions. How can such a feature be explained? From a compositional point of view, wood hydrolysate differs from refined AcGGM in that the wood hydrolysate contains 91% polysaccharides, the lion’s share of which is AcGGM, and about 9% lignin according to SEC.19 Scheme 1 shows the representative structure of AcGGM and the major monolignol building blocks of lignin. In the native state, strong interactions between lignin and hemicelluloses obviously exist and it has been proposed that these include covalent interactions, at least to a fair extent. Such lignin-carbohydrate complexes (LCCs) are believed to consist mainly of R-ester, Rether or phenyl glucosidic linkages.17 A hemicellulose like galactoglucomannan has been reported to bind at the R-position of monolignols37 although the relative occurrence of the proposed LCCs is not known. To acquire a fundamental picture of how the coexistence of lignin and oligo- and polysaccharide components, and the preservation of their native primary and secondary interactions, in the wood hydrolysate synergistically provide a better oxygen barrier we need to probe their interaction driven compatibility and the resulting morphology on a molecular length scale. The ternary system characteristics arising from co-formulations of wood hydrolysate and CMC or chitosan need to be probed accordingly. To date, positron annihilation lifetime spectrum (PALS) is the only technique that quantifies the nanostructural 1358

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Table 2. Hansen’s Solubility Parameters δD, δP, δH, and δ for Pure AcGGM, Wood Hydrolysate, CMC, and Chitosan and the Interaction Factor Ra δD

δP

δH

δ

(MPa1/2) (MPa1/2) (MPa1/2) (MPa1/2) lignin

a

21.9

14.1

16.9

31.0

Ro

Table 3. ortho-Ps (o-Ps) Lifetime Parameters: the o-Ps Intensity I3, the Mean o-Ps Lifetime τ3, and Standard Deviation of o-Ps Lifetime Distribution r3(τ); σ3 are Listed for Films Based on Wood Hydrolysate or AcGGM with a Co-Component Being Chitosan or CMC film compositiona

13.7

I3b (%)

τ3c (ns)

σ3d (ns)

AcGGM (DSAcb = 0.6)

17.9

12.8

23.8

32.4

wood hydrolysate þ CMC

18.8

1.204

0.276

CMC

18.7

13.0

24.0

33.1

chitosan AcGGM (DSAc = 0.3)

17.8 18.3

14.2 14.0

24.1 26.5

33.2 35.1

wood hydrolysate þ chitosan AcGGM þ CMC

17.8 22.1

1.279 1.349

0.261 0.333

AcGGM þ chitosan

21.5

1.390

0.294

wood hydrolysate c

18.3

12.9

23.2

32.3

Distance of Each Component in Hansen’s Space Ra (lignin-AcGGM)

10.6

Ra/Ro(lignin AcGGM)

0.78 < 1

Ra (wood hydrolysate-CMC)

1.2

Ra (wood hydrolysate-chitosan)

1.8

Ra (AcGGM-CMC) Ra (AcGGM-chitosan)

2.8 2.6

a Values from Hansen’s handbook.34 b Degree of acetyl substitution; DSAc. c Wood hydrolysate values were calculated from a AcGGM/lignin ratio of 9.1:0.9 and with the AcGGM having a DSAc of 0.6.

morphology in terms of free volume size and distribution in disordered systems like the current one. A denser molecular packing would provide a lower permeability of small permeants like oxygen through the matrix and a denser packing, in turn, forms from polymer segments of high mutual affinities which locate near each other in the resulting matrix. This mutual affinity, the interaction driven compatibility of components in a mixture, can be estimated by various techniques. The Hansen’s solubility parameters (HSP) concept34 has been utilized for a long time as a practical tool for estimation of component affinity based on their cohesive energies, respectively. We used a group contribution calculation and the HSP concept as elaborated in the Experimental Section to estimate theoretically the disperse parameter δD, polar parameter δP, and hydrogen-bonding parameter δH. Table 2 shows the results of calculated HSP parameters δD, δP, δH, and δ for the repeat units of AcGGM, CMC, chitosan, and wood hydrolysate. The estimated distance of lignin and AcGGM in the Hansen’s space, Ra(lignin-AcGGM), is 10.6, while the radius of interaction sphere of lignin in HSP space, Ro (lignin), is 13.7. Hence, Ra/Ro < 1.0, putting AcGGM just inside the interaction sphere of lignin in the HSP space. We also discovered in calculation that the higher the degree of acetyl substitution, the stronger tendency of mutual affinity of AcGGM with lignin. This indicates a close interaction between lignin and AcGGM in wood hydrolysate and their mutual affinity is a strong driving force for packing their segments together tightly, resulting in an immobilization of the chains in a dense disordered structure. This, together with the stiff ring structures of lignin, can effectively lower the internal voids or holes in the structure, lowering the free volume resulting in a lower permeability as compared to AcGGM based system. When including a co-component in the matrix, Ra (wood hydrolysate-CMC) becomes 1.2, while Ra (wood hydrolysatechitosan) is 1.8. The smaller value translates to a smaller distance between wood hydrolysate and CMC in the HSP space

The films used here all have a concentration of 0.4/14 mL and a composition of resource to co-component to be 1:1. b The standard error for I3 is (1%. c The standard error for τ3 is (0.03 ns. d The standard error for σ3 is (0.04 ns. a

compared to wood hydrolysate and chitosan and, thus, a higher affinity of the former system. The better compatibility between wood hydrolysate and CMC suggests a stronger intermolecular interaction in this case. This interpretation is in accord with the OTR and OP values, verifying that a better oxygen barrier is built from wood hydrolysate and CMC rather than wood hydrolysate and chitosan. For the purified AcGGM matrix, Ra (AcGGM-CMC) is very similar to Ra (AcGGM-chitosan), and the compatibility of AcGGM with the co-components is essentially equal. This is reflected in the OTR results for AcGGM-CMC and AcGGMchitosan, which are quite similar at both 50% and 80% RH. This theoretical approach identifies some coating compositions as having an inherently higher driving force for closer molecular packing based on their higher mutual interactions: wood hydrolysate rather than purified AcGGM should be used as the base component and CMC rather than chitosan as the cocomponent. When PALS is used, the true molecular packing can be quantified with respect to size and distribution of free volume from the lifetime, τ3, of positronium in its ortho-state, o-Ps. PALS measurements provided positron lifetime spectra for films based on wood hydrolysate or AcGGM co-component with CMC or chitosan. The spectra have a good fit with reduced Chisquares (the variance of fit) of ζ2/df = 1.00-1.03, which suggests the model LifeTime, version 9.0 (LT 9.0) describes the experiments well. From the lifetime spectra, o-Ps parameters are calculated and summarized in Table 3. The o-Ps intensity, I3, reflects the probability of formation of oPs in the material, which is related to the number density of the holes. The films based on wood hydrolysate typically show lower values of I3, indicating that there are less o-Ps formed in wood hydrolysate based films than AcGGM based films. Mean values for all the films lie in between 1.204 and 1.390 ns. The τ3 is in the range of 1 to 8 ns for amorphous polymers, depending on the differences in structure and temperature. Thus, all the films investigated here have relatively short lifetimes, which indicates smaller free volume (holes) than amorphous materials in average and have a denser molecular packing. Wood hydrolysate has a shorter τ3, around 1.2 ns, while τ3 of AcGGM based films is in the order of 1.35 ns, which suggests that smaller hole sizes exist in the matrices of wood hydrolysate based films. It can also be noticed that CMC, as a co-component, has a tendency to produce more densely packed structures. The standard deviation of o-Ps lifetime distribution R3(τ) reflects more or less the distribution of hole sizes. Wood hydrolysate based films have a smaller hole size distribution than AcGGM. 1359

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Figure 1. Hole radius distribution n(rh) (probability density function, pdf) for films produced by AcGGM and chitosan (solid line), AcGGM and CMC (dash), wood hydrolysate and chitosan (dot), and wood hydrolysate with CMC (dash-dot) at a concentration of 0.4 g/14 mL and ratio of 50/50 (w/w). The distributions were normalized to the unity area below the curve.

The free volume size (hole) and distribution, hole radius and hole volume distribution for the films prepared herein are shown in Figures 1 and 2, as measured by PALS and calculated by eqs 4 and 5a. The prepared films can in average be characterized as very densely packed with fairly small holes, in the range of those found in the disaccharide trehalose.38 As expected from the calculations based on HSP, the interaction of lignin and hemicellulose AcGGM in wood hydrolysate encourages molecules to pack more closely when forming a film with a second polysaccharide co-component. Wood hydrolysate based films give not only a smaller free volume size, quantified as hole radii and hole volumes, but also a smaller free volume distribution than corresponding AcGGM based films (Figures 1 and 2). The hole radii of wood hydrolysate based films are 1.9 and 2.05 Å, while the corresponding values for AcGGM based films are 2.1 and 2.2 Å. This parallels the observed differences in permeability, where wood hydrolysate based coatings consequently show a lower OP values than AcGGM coatings. Similarly, as a co-component, CMC was shown to be more compatible with AcGGM and wood hydrolysate than the other co-component by the HSP calculation. PALS measurements indicate that CMC-based matrices have smaller free volume size and distribution than chitosanbased ones. The coating displaying the most dense structure is the wood hydrolysate/CMC film, with a hole radius of 1.9 Å and a hole volume of 24 Å3 (volume fraction) and 12 Å3 (number fraction). Accordingly, it is also the coating which shows lowest OP values, both at medium and high relative humidities. The theoretical approach and the PALS data both sustain the differences in matrix component interactions between the hydrolysate versus purified AcGGM on one hand and the nature of the co-component on the other hand and furthermore that the interactions are functional in providing a change in permeability properties of the matrix. According to the HSP calculation, hydrogen bonding offers the strongest contribution to the matrix cohesive energy among the three parameters. Hydrogen bonds are thus an important part of the interactions between hydrolysate components, which seems reasonable considering their structures. The changes in hydrogen bonding following hydrolysate/co-component formulation are visualized by FTIR. Figure 3 shows that the hydroxyl groups in the wood hydrolysate form hydrogen bonding, which

R Figure 2. Hole volume distribution normalized to g(vh)dvh = 1 for (a) g(vh), giving the volume fraction of holes of sizes between vh and vh þ dvh and (b) gn(vh), giving the fractional number of holes of sizes between vh and vh þ dvh for for films produced by AcGGM and chitosan (solid squares), AcGGM and CMC (solid triangles), wood hydrolysate and chitosan (open rings), and wood hydrolysate with CMC (open triangles) at a concentration of 0.4 g/14 mL and ratio of 50/50 (w/w).

gives rise to a broad peak at 3348 cm-1, while the corresponding broad peak in AcGGM is at a slightly higher wavelength (3368 cm-1). The shift of the hydroxyl band indicates a partial break-up of internal O-H interactions and a weakening of the O-H bond strength caused by competing hydrogen bonding to adjacent species capable of forming hydrogen interactions.39 Thus, wood hydrolysate possesses more hydrogen bonding interactions, partly contributed by intramolecular interaction between lignin and AcGGM in wood hydrolysate. This interaction can be one of the driving forces that contribute to the affinity between lignin and AcGGM, as calculation results show in the HSP theory. In the case of pure CMC, this hydroxyl group stretching appears at 3253 cm-1. When combining CMC and wood hydrolysate this peak is shifted to 3292 cm-1, a shift in hydrogen bonding interactions that can be reasonably assumed to result from hydrogen bonding between CMC and wood hydrolysate, compared with the OH stretches of individual components. Chitosan films give rise to a broader band peaking at 3361 cm-1, which is an overlaid peak stemming from NH and OH group stretches. In the same region, for films of wood hydrolysate and chitosan, this broad band is shifted slightly to 3357 cm-1. In addition, pure chitosan gives rise to a sharp peak at 1587 cm-1, representing a N-H deformation vibration. This peak slightly shifts to a higher wavenumber, 1592 cm-1, upon blending with the hydrolysate, indicating that hydrogen bonding increases in the system. This increase is very subtle, not as noteworthy as for the wood hydrolysate/CMC system, which causes a 100 cm-1 shift. Pure AcGGm as well as the wood hydrolysate give rise to a carbonyl peak at 1726 cm-1 stemming from the acetyl pendant groups on the glucomannan backbones. 1360

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Figure 3. FTIR spectra of (a) AcGGM, (b) wood hydrolysate, (c) films of wood hydrolysate and CMC in a 50/50 ratio (w/w), (d) CMC, (e) films of wood hydrolysate and chitosan in a 50/50 ratio (w/w), and (f) chitosan.

Figure 4. Thermogravimetric analyses of films composed of (a) CMC, (d) wood hydrolysate, and films based on wood hydrolysate-CMC at a concentration of 0.4 g/14 mL, with ratio of wood hydrolysate to CMC to be (b) 50/50 (w/w) and (c) 80/20 (w/w).

Figure 5. Thermogravimetric analyses of films composed of (a) chitosan, (d) wood hydrolysate, and films based on wood hydrolysatechitosan at a concentration of 0.4 g/14 mL, with ratio of wood hydrolysate to chitosan to be (b) 50/50 (w/w) and (c) 60/40 (w/w).

These shifts support the hypothesis that there are intermolecular interactions created between the wood hydrolysate and the selected co-components that can help explain the verified increased molecular packing and the observed decrease in oxygen permeability. The hydrogen bonding is more pronounced when using CMC as the co-component which is consistent with the smaller distance between CMC and wood hydrolysate in the Hansen’s spaces, and hence an improved compatibility, as previously described in the calculations. The matrix interaction is also reflected in the thermal stability behavior of hydrolysate. According to thermogravimetric analyses, summarized in Figures 4 and 5, the onset of thermal degradation is 170
C for the wood hydrolysate, 300
C for CMC, and 250
C for chitosan. Wood hydrolysate/CMC films show an onset of degradation at about 210
C, before which they show a better thermal stability than either of the individual components alone prior to the onset of degradation. Similarly,

wood hydrolysate/chitosan films exhibit improved stability, with the onset of decomposition at about 150
C. This shows that the intercomponent compatibility results in a strongly interacting system, which improves the initial thermal integrity of the films. According to the HSP calculations, wood hydrolysate is more compatible with CMC, thus, a wood hydrolysate-CMC composite is thermally stable up to a higher temperature than a wood hydrolysate-chitosan composite.

’ CONCLUSIONS Renewable barrier films were prepared from a wood hydrolysate, a polysaccharide rich but not highly refined mixture where a fair amount of the native intermolecular and intramolecular hemicellulose-lignin interactions are purposely preserved. A proper balance of the network of interactions between constituent molecules is further engineered by adding a co-component, 1361

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Biomacromolecules CMC or chitosan. The barriers, coated on a polyethylene terephthalate substrate, show very low oxygen permeabilities (OP) at both 50 and 80% relative humidities, and show considerably lower OP values than coatings based on the highly purified spruce hemicellulose, O-acetyl galactoglucomannan (AcGGM). The best composition, investigated herein is a wood hydrolysate/CMC coating, which reduces the oxygen permeability of PET from 14.6 cm3 μm m-2 day-1 kPa-1 to as low as 1.6 cm3 μm m-2 day-1 kPa-1. Group contribution calculations and Hansen’s solubility parameter concept (HSP) confirm that the wood hydrolysate components show strong mutual affinities and that the CMC cocomponent is closer to the wood hydrolysate in the HSP space than chitosan, suggesting a higher affinity. These mutual affinities effectively mediate an immobilization of the chain segments in a dense amorphous structure as quantified on the nanolength scale by positron annihilation lifetime spectrum (PALS), showing that wood hydrolysate based films are densely packed systems with a free volume average void radius distribution of Ærhæ = 2-2.2 Å and mean number-weighted hole volumes of 27-31 Å. Wood hydrolysate films are more densely packed that those prepared from the highly purified hemicellulose and the CMC co-component gives rise to denser matrices than chitosan. Denser molecular packing correlates exactly with the decrease in OP values of wood hydrolysate based coatings and is also reflected in an increase in thermal stability as measured by TGA.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge VINNOVA (Project No. 2009-04311) and Tetra Pak Packaging Solutions AB for financial support. Dr. Margaretha S€oderqvist-Lindblad at S€odra Innovation AB is thanked for kindly providing the wood hydrolysate and Dr. Olof Dahlman at Innventia AB is kindly thanked for ultrafiltration of the hydrolysate. The author would like to also acknowledge Prof. G€unter Dlubek and Prof. Reinhard KrauseRehberg from the University of Halle in Germany for assistance with the PALS measurement. ’ REFERENCES (1) Zhang, P. Y.; Whistler, R. L. J. Appl. Polym. Sci. 2004, 93 (6), 2896–2902. (2) Hartman, J.; Albertsson, A. C.; Sj€oberg, J. Biomacromolecules 2006, 7 (6), 1983–1989. (3) Hartman, J.; Albertsson, A. C.; Lindblad, M. S.; Sjoberg, J. J. Appl. Polym. Sci. 2006, 100 (4), 2985–2991. (4) Goksu, E. I.; Karamanlioglu, M.; Bakir, U.; Yilmaz, L.; Yilmazer, U. J. Agric. Food Chem. 2007, 55 (26), 10685–10691. (5) Hansen, N. M. L.; Plackett, D. Biomacromolecules 2008, 9 (6), 1493–1505. (6) Mikkonen, K. S.; Heikkinen, S.; Soovre, A.; Peura, M.; Serimaa, R.; Talja, R. A.; Helen, H.; Hyvonen, L.; Tenkanen, M. J. Appl. Polym. Sci. 2009, 114 (1), 457–466. (7) Krawczyk, H.; Persson, T.; Andersson, A.; Jonsson, A. S. Food Bioprod. Process. 2008, 86 (C1), 31–36. (8) Lindblad, M. S.; Albertsson, A. C.; Ranucci, E. ACS Symp. Ser. 2004, 864, 347–359.

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